The invention herein relates particularly to improvements in rotary electrical switches and particularly of the type set forth in U.S. Pat. No. 4,864,083 assigned to Lucerne Products, Inc., the assignee of the instant application. That patent teaches a particular reversible rotary switch of the type under consideration herein. It has been found that reversing switches of the type known in such prior art often demonstrate an intermittency in the switching operation, characterized by unwanted making and breaking of the contacts. It has been found that rocking or otherwise manipulating the reverse lever of the switch may cause such intermittent operation. Upon inspection, it was found that the forces applied to the moving contacts of the reversing switch cause a rocking motion to occur rather than a pure sliding motion of the movable contacts over the stationary contacts.
An appreciation of the problems encountered in the prior art may be obtained by reference to the prior art switch of FIG. 1. There, a reversing switch according to the prior art is designated generally by the numeral 10. Switch 10 includes a rotor 12 adapted for rotation about an axis 14 by actuation of a reversing lever 16, shown only illustratively. For a complete understanding of the operation of the switching mechanism, reference need simply be made to prior U.S. Pat. No. 4,864,083.
Maintained as a portion of the reversing switch 10 is a first pair of stationary contacts 18 and a second pair of stationary contacts 20. As shown in FIG. 1, the contacts of the pair 18 are diametrically opposed from each other, as are the contacts of the pair 20. The diametric opposition is taken with respect to the rotational axis 14. Received upon the diametrically opposed ends of the rotor 12 are a pair of movable contacts 22. Those skilled in the art will readily appreciate that the movable contacts 22 are typically U-shaped and are slidingly received upon the opposite ends of the rotor 12 in recesses 28 provided for such purpose.
Springs 24 are provided in appropriate pockets or bores 26 in the rotor 12, such springs 24 biasing the respective movable contacts 22 axially from the rotational axis 14 to urge the movable contacts 22 toward the stationary contacts 18, 20. It will be readily appreciated by those skilled in the art that springs 14 contact a portion of the rotor 12 at one end thereof and extend outwardly therefrom and into contact with an associated movable contact 22 at the other end.
The operation of the reversing switch 10 is fully set forth in U.S. Pat. No. 4,864,083. Suffice it to say that a reversing lever 16 is operative to rotate the rotor 12 about the axis 14 to bring the movable contacts 22 into interconnecting engagement with selected ones of the pairs of stationary contacts 18, 20. The rotor 20 is adapted for reciprocating movement between the pairs of contacts such that in one position standard or forward operation of the associated motor is attained, while in the opposite position, reverse operation of the motor is attained. It is, of course, contemplated that the movable contacts 22 may also be positioned intermediate the positions just stated, such that the movable contacts 22 engage only the stationary contacts 18. Such position is typically a neutral or locked position. Again, those skilled in the art will appreciate that the rotational movement of the rotor 12 is achieved by movement of a reversing lever keyed thereto as at 16.
In the desired standard operation of the switch 10, it is contemplated that the movable contacts 22 will smoothly slide upon the stationary contacts 18, 20 and that the movable contacts 22 will stay longitudinally aligned upon the rotor 12. Consequently, each of the movable contacts 22 will be in constant contacting engagement with diametrically opposed pairs of the pairs of contacts 18, 20 or in the neutral position, in engagement with only the stationary contacts 18. However, it has been found that such consistent contacting engagement is not always attained in the prior art structure.
With continued reference to FIG. 1, it can be seen that a counterclockwise motion of the rotor 12 has been commenced by movement of the reversing lever 16, as designated by the rotational arrow 30. As shown in FIG. 1, the lower movable contact 22 stays in sliding engagement with the stationary contacts 18, 20, while the upper movable contact 22 has rocked from its longitudinal alignment upon the rotor 12 and, while remaining in engagement with the stationary contact 18, has rocked away from the stationary contact 20, providing a gap 38 therebetween. The gap 38 is developed because the frictional engagement between the associated movable contact 22 and stationary contact 18 is not overcome by the rotational force provided to the rotor 12 and because the force to rock the contact 22 from alignment with the rotor 12 is less than the force of such frictional engagement.
As shown in FIG. 1, a force 32 is provided to the rotor 12 by means of the reversing lever 16. An opposite frictional force 34 is developed at the interface between the contacts 22, 18, as shown. The forces 32, 34 are separated by a distant d designated by 36. Accordingly, a force couple is generated on the associated moving contact 22, causing the same to rotate as shown in FIG. 1, become misaligned upon the rotor 12, and generating the gap 38. In other words, the moment or force couple generated by the forces 32, 34 separated by the distance 36, causes the rotational movement of the movable contact 22 and the resultant gap 38, accounting for the undesired intermittency of operation.
The happenstance of the intermittence just discussed is a function of the spring constant of the associated springs 24 and the coefficient of friction at the interface between the associated movable contact 22 and the stationary contact 18. Since the static coefficient of friction is generally greater than the dynamic coefficient of friction, typically the static coefficient of friction is of primary concern.
Since it appears that a frictional engagement between the stationary and contact and moving contact develops the moment or force couple which rocks the movable contacts and causes the intermittence of operation, it is desirable to minimize or eliminate the force couple. Since a moment or force couple is the product of force and distance, either the force or the distance must be minimized or substantially eliminated to achieve the desired object. Since the frictional force is a function of the normal force at the interface and the characteristic coefficient of friction, the frictional force can be reduced only by reducing the normal force. However, the normal force is that imparted by the springs 24, which also provide for assurance of positive engagement between the movable and stationary contacts, while also assuring the necessary current conducting capacity. Accordingly, it is not desirable to alter or reduce the spring force of the reversing switch 10.
Absent a means for reducing the frictional interference between the moving and stationary contacts and/or the resultant force couple, complex and expensive manufacturing, sorting, and testing procedures have been found to be necessary to minimize the intermittent problem. Not only is it desirable to eliminate such procedures, but it is also desirable to provide for a means for eliminating the intermittency problem with a switch design which may be easily manufactured.